366 ON THE CHANGE OF REFHANOnHLITY OF LIGHT. reduced to one-half, one-third, &c., or doubled, trebled, &c. This comes to the same thing as supposing that each absorbing molecule stops the same fractional part of the light passing it, whether the solution be more or less dilute. We should similarly be inclined to suppose that each sensitive molecule would give out the same quantity of light, when influenced by light of given intensity, whether it belonged to a stronger or a weaker solution. If we admit these suppositions, it is plain that the quantity of dispersed light which reaches the. eye from the element under consideration will be independent of the strength of the solution. This being true for each element in particular will be true for the aggregate effect of them all, and therefore the quantity of light exhibited by dispersive reflexion will be independent of the strength of the solution. It may he readily seen that the result will be the same if we take into account the finite size of the pupil. 187. Now this is by no means true in experiment. On examining in a pure spectrum a highly concentrated solution of sulphate of quinine, a copious dispersion was observed to commence a little below the fixed line (}. It remained very strong as far as If, and beyond. In the weak solution first mentioned in this paper, it will be? remembered that the dispersion seemed to come on about G\H. The reason of this, or at least one reason, is evident, and was very prettily shown by the form of the space to which the dispersed light was confined. fill On looking down from above, so that this spare was seen in projection, it appeared in the case of the weak solution to have approximately the form of the space contained between one branch of a rectangular hyperbola, one, asymptote, and a, line parallel to the other, the first asymptote being the projection of the anterior surface, and the, line parallel to the other being the course of the least refrangible of the active; rays which were capable of producing a sensible quantity of dispersed light. The breadth of the illuminated space, which among the most highly refrangible rays was almost insensible, continually increased, until the space ended in a blue beam which went quite across the vessel. But in the case of the strong solution the illuminated space had throughout an almost insensible breadth, exw.pt just close to its lower limit, that is, the limit corresponding to the